1. Field of the Invention
This invention relates to a superconducting wire and a manufacturing method thereof.
2. Description of the Related Art
When manufacturing a superconducting wire in which a copper oxide superconducting material is covered with a metal, a single core is generally made by filling oxide powders in a metallic pipe of, for example, silver. The single cores are bundled together, and are inserted into another metallic pipe of, for example, silver, thereby obtaining a multi-core structure. A bus having the multi-core structure undergoes wire drawing or rolling so as to become wire-shaped, and then undergoes heat treatment. As a result, a wire provided with superconductivity can be obtained. In the superconducting wire manufactured in this manner, there are three important points required for its practical use as follows:
(1) Superconducting wire has a high critical current value;
(2) Superconducting wire does not have the defect of, for example, bulging; and
(3) Superconducting wire does not undergo a decrease in the critical current value (with increased mechanical strength) when various external forces are applied thereto.
Concerning the first point (1), the critical current value depends greatly on how powder portions having superconductivity are transformed (reacted) to a desired superconducting phase with high purity. Since a superconductor is an oxide, a chemical exchange is inevitably carried out between the oxide and outside gas (especially, oxygen) in the reaction. Accordingly, in order to obtain a high purity superconducting phase, it is important to cause gas (especially, oxygen) to move in and out smoothly (almost naturally).
Concerning the second point (2), bulging, which is associated with the reaction of the oxide, occurs such that oxygen gas emitted from the powders during the reaction accumulates in the wire and expands due to heat treatment, thus bulging the wire.
In order to achieve points (1) and (2), a metal-covered oxide superconducting wire uses silver as a covering material. Silver is high in oxygen permeability and allows oxygen to flow in and out smoothly between the inside of the wire and the outside atmosphere. In addition, there has been employed a method of decreasing the thickness of a silver covering or forcibly boring holes through the silver covering so as to facilitate the in and out movement of oxygen. This method of boring holes therethrough, is disclosed by, for example, Japanese Laid-open Patent Publication No. Hei-1-161623.
Concerning the third point (3), when using silver as a covering material, a method of adding second and third metals to the silver has been generally employed to improve the strength of a covering because pure silver is weak in mechanical strength. In other words, in a case where silver to which second and third metals have been added is used as a starting material (called covering tube or alloy tube), such additives are oxidized during heat treatment such that they are dispersed as oxides in the covering. As a result, the strength of the covering is improved This method is disclosed in, for example, Y.Hikichi et al., xe2x80x9cDevelopment of Agxe2x80x94Mgxe2x80x94xcex1 Alloy Sheathed Bi2223 Multifilament Tapesxe2x80x9d, Advances in Superconductivity XI, Nov. 16-19, 1998, Fukuoka, pp.915-918.
However, it is difficult for conventional techniques to satisfy the three points (1), (2), and (3) simultaneously. The reason is that the method of thinning the covering or forcibly boring holes for achieving the points (1) and (2) lowers the mechanical strength of the covering. The mechanical strength of the covering increases with the increase of the volume of the covering, but the strength thereof decreases because the volume of the covering decreases because of thinning the covering. Further, if holes are locally bored through the covering, the local strength of the covering is greatly lowered, and as a result, the average strength thereof decreases.
On the other hand, the method employing a silver-covered tube to which the second metal is added to heighten the strength thereof has the following drawback.
An oxide superconductor, especially a bismuth-based superconductor is seldom reactive to silver, and is easily and smoothly shifted to a desired superconductive phase when it is in contact with silver. However, it is easily reactive to elements (components) other than silver, and, accordingly, it causes a compositional disarrangement of the powder portion and cannot obtain the desired high purity superconductive phase. Thus, the critical current of a wire using the alloy tube was lower than that of a wire using a pure silver tube.
It is therefore an object of the present invention to provide a superconducting wire that has a high critical current value and no defects of bulging for example, and, additionally, has a high mechanical strength, and to provide a manufacturing method thereof.
A superconducting wire according to the present invention comprises an oxide superconducting material; a metallic covering material made of metal, for covering a surface of the oxide superconducting material; and ceramic materials buried in a surface of the metallic covering material.
In the superconducting wire of the present invention, ceramic materials are buried in a metallic covering from the outer surface of the covering, and thereby the outer layer of a part of the covering where the ceramic materials have been buried thins partially. In addition to this, the contact interface between the buried ceramic materials and the metallic covering serves as a passage through which gas easily flows in and out. This provides the same effect as the conventional technique of thinning. Accordingly, a powder portion having superconductivity can be changed to a superconductive phase with high purity, and, additionally, defects such as bulges can be reduced.
Concerning the thinning, ceramic materials are structurally dispersed into the metallic covering, thus improving the mechanical strength advantageously.
Since the ceramic materials are buried from outside in the outer surface of the metallic covering, the contact probability between the ceramic materials and the superconducting powder portion is small, and the powder portion does not facilitate the disarrangement of its composition. Accordingly, a high purity superconductive phase can be obtained.
These make it possible to obtain a superconducting wire that has a high critical current value, no defects such as bulges, and a high mechanical strength.
Preferably, ceramic materials in the superconducting wire are in a form of at least particles or fibers.
Thus, in the form of particles or fibers, gas can easily flow in and out, and the mechanical strength is heightened.
Preferably, the ceramic materials in the superconducting wire, are oxides. Accordingly, it is possible to perform a method in which the surface of the metallic covering is plated with metal elements of the ceramic materials and, in a subsequent heat treatment process, the metal elements are changing to oxides as ceramic materials.
More preferably, the ceramic materials in the superconducting wire comprise one or more kinds of materials selected from the group consisting of alumina, magnesia, and zirconia. Accordingly, particularly high critical current values can be obtained, and a material suitable for the ceramic material can be selected as necessary.
Preferably, the oxide superconducting material of the superconducting wire comprises a bismuth-based superconductor.
Thus, the bismuth-based oxide superconductor is suitable for the structure of the present invention.
The material of the metallic covering material of the superconducting wire mainly comprises silver. This enables oxygen to flow in and out smoothly when the oxide superconducting material is changed to a superconductive phase with high purity, and the reaction between the metallic covering and the oxide superconducting material can be prevented.
In a method of manufacturing a superconducting wire of the present invention, heat treatment is conducted under a state in which a surface of a member including at least a superconductive phase is covered, and ceramic materials are buried in a surface of a metallic covering material.
Accordingly, the superconducting wire manufacturing method of the present invention enables the manufacture of a superconducting wire having a high critical current value, no defects such as bulges, and a high mechanical strength. Further, since the ceramic materials do not react easily with an oxide superconductor, the oxide superconducting material can be shifted smoothly to a superconductive phase of an oxide superconducting material even when heat treatment is conducted while the ceramic materials are buried in the surface of the metallic covering.
Preferably, in the manufacturing method, the process of burying the ceramic materials in the metallic covering material comprises a step of mechanically pressing the ceramic materials and the metallic covering material while the ceramic materials are dredged over the surface of the metallic covering material.
Accordingly, the ceramic materials can be buried in the metallic covering material.
Preferably, the step of mechanically pressing method comprises one or more methods selected from extruding, drawing, rolling, and pressing.
Accordingly, the step of mechanical pressing the ceramic material to bury it in the metallic covering material can be selected appropriately.
Preferably, in the manufacturing method, the process of burying the ceramic material in the metallic covering comprises a step of conducting heat treatment while ceramic materials contact the surface of the metallic covering material.
Accordingly, it is possible to perform a method of plating the surface of the metallic covering with metal elements (for example, Aluminum, Magnesium, and Zirconium) of the ceramic materials and, in a subsequent heat treatment process, changing the metal elements to oxide ceramic materials.